The present invention relates to an apparatus for the in-circuit detection of defective capacitors of the type used on electric power transmission and distribution systems, and particularly of those installed in large numbers as a bank, at a substation in the electric power distribution system.
The shunt connected high voltage distribution type capacitor units are used extensively by the electric utility industry and manufacturing industry in their electrical supply systems for correcting the power factor of the system and maintaining proper range of voltage levels. A common method of construction of these capacitors consists of two sheets of pure aluminum foil separated by layers of kraft paper or plastic film impregnated with chromated aromatic hydrocarbon compounds. A capacitor unit is made up of several of these individual small sized capacitor packs or rolls, which are connected in a parallel-series arrangement in order to obtain the desired capacitance value and withstand the high voltages in the range of 2,000 volts to 20,000 volts. The capacitor unit is enclosed in a sealed metal can, having one or two insulated bushing-type terminals extending from the top. If desired, the second bushing may be omitted, in such a case, the metal can is interconnected with the neutral bus floating mid-point or ground in the system.
The metal can is filled with an insulating liquid. Before 1977, most capacitor cans contained PCB-type non-flammable insulating fluids. However, such materials have been condemned because of excessive toxicity and environmental problems and the more recently, the insulating fluids that have been utilized in place of PCB-type fluids are flammable. Generally, the capacitor cans are connected in the electric transmission and distribution system at various locations in small groups or units close to the load centers, or in much larger groups or banks at substations in the system.
It is important that as many of the capacitor units as possible in an electric transmission and distribution system be in a serviceable condition so that during periods of high electrical load, the system voltage is held within a prescribed range. However, because of periodic failures within the large banks of capacitors installed at substations, at any given time there may be a large number of capacitor units disconnected from the system and awaiting service. In order to compensate for these failed capacitor units, either a surplus of capacitor units over and above the minimum necessary is required, or the generators must supply the additional reactive power at the expense of producing real power. In either case, the result is an unacceptable additional monetary expense.
Prior to the present invention, detection of failed capacitor units by electric utility workmen has been primarily that of a visual observation. A blown fuse is one signal to a workman that one or more capacitor units is out of service. If the workman cannot visually detect a sign of failure, such as a bulged metal can or a leaking seam or bushing, the operating practice has been to re-energize the capacitor unit with a new fuse.
However, the above practice has very serious drawbacks and limitations. Firstly, should the capacitor bank be re-energized when a capacitor unit actually has an electrical short, for which no visual indication is apparent, there is the increased probability of a violent rupture (affecting many adjacent capacitor units) and the release of the dielectric fluid onto the immediate area. Containment of the liquid dielectric fluid within the capacitor can is very important because the capacitor units, containing PCB-type insulating fluids, must not be allowed to contaminate the immediate area. Such contamination results in expensive and time-consuming cleanup procedures and if a violent rupture should occur when workmen are in the vicinity of such a rupture, severe injuries may occur. Additionally, the non PCB-type fluids are flammable and present a fire hazard upon failure.
Secondly, a capacitor unit may have a partial or incipient fault because one or more of the small series group connected packs, making up the total capacitor unit, has shorted or opened. The capacitor unit may withstand the applied line voltage for a short period of time, perhaps hours or weeks, by a redistribution of line voltage across the remaining operative series connected capacitor packs. However, arcing within the shorted capacitor pack or between adjacent capacitor packs may cause the dielectric fluid and insulation to decompose into hydrocarbon gases, thereby bulging the capacitor can and creating a potentially explosive condition when the eventual electrical failure of the capacitor unit occurs. Field experience has shown that once a capacitor is beyond its infancy period, the predominance of field failures are pack failures.
In addition, large installations of capacitors at substations in the power transmission and distribution systems are often connected in a ungrounded wye or three phase configuration, each leg of the wye having many capacitors connected together in a series-parallel arrangement. If too many capacitors fail on one leg of the wye as compared to the other two legs, a shift in voltage will occur resulting in a higher than normal voltage across the leg of the wye having the excess number of failed capacitors. This in turn results in more failed capacitors due to the overvoltage and a possible runaway condition which will trip the entire capacitor bank off the energized bus. For these reasons, it is important that the condition of the capacitors installed in large banks in substations be known.
There have been several suggested methods and apparatuses for determining the condition of a capacitor unit, however, these suggestions all require operations which are beyond the assigned duties, and technical capabilities of the average lineman or mechanic. For example, the USA Standards for Shunt Power Capacitors 0551-1968, describes various field tests for these type capacitor units. One such test requires the application of a known voltage and frequency of undistorted wave shape to the capacitor unit. However, this test of capacitance measurement requires disconnecting each capacitor unit from the remainder of the bank and then individually measuring the current drawn by each unit. Such tests require a high degree of technical competence to interpret the resultant meter readings and the manpower cost associated with such tests are prohibitive. Moreover, the disconnecting and handling of the capacitor unit may indeed cause damage to the capacitor unit which results in incipient faults.
Also, commercially available capacitance meters, which apply a low AC voltage to provide a direct capacitance reading of the capacitor are unreliable on paper film capacitors. The gases created by arcing in a defective unit insulates the defective pack to the voltage applied by the capacitance meter and, therefore, capacitance readings obtained may be in error. It has been observed that for an applied low voltage on a capacitance bridge, that certain known shorted capacitors were erroneously measuring as good units. Such a result indicated that a certain minimum level of voltage was required to break down the gas bubbles adjacent to faulted packs. These particular capacitors were paper film PCB-type liquid units. Repetitive tests showed breakdown of 7,200 volt units at voltages up to 76 volts AC. Use of this device required disconnecting each unit in the capacitor bank during testing, which results in large manpower costs and expensive time loss during testing of the capacitors.
In a previously filed application, U.S. Ser. No. 301,949 filed Sept. 14, 1981 and U.S. Ser. No. 576,783 filed concurrently herewith, by the inventors of the present invention, the inventors have described apparatus which was optimally designed for the detection of faults on capacitors which are installed in groups of two to four on utility poles and protected by a single fuse. The apparatus for the detection of capacitor faults, either incipient or fully developed, when installed in the smaller groups of two-four per phase on utility poles, utilized a transmitter having a power frequency in the order of 50/60 to 800 hertz in order to drive sufficient electric current through capacitors to allow accurate measurement of the charging current at a nominal 120 volts. Because the power source was a portable or a mobile battery/inverter, the economics and practical size limit the applicability of the apparatus to capacitor groups of about four per phase, which is normally sufficient for pole mounted capacitor installations. In addition, because of the method of mounting pole type capacitors, the split-core detection device must be fitted over the large diameter procelain insulator bushings of the capacitor. These applications disclose a sensitive split-core Hall-Effect or special distributed winding split-core detection device. However, a substation capacitor bank size, generally may consist of several hundred cans, with between about 5-40 capacitor units in parallel in a phase to be tested at one time, makes a battery/inverter transmitter impractical and non-economical because of power output requirements.